Downing Street has admitted that it is powerless to force the return of the Lockerbie bomber to Britain almost two years after the Libyan was released from a Scottish prison on compassionate grounds.

David Cameron's official spokesman said that there was "no mechanism" to return Abdelbaset al-Megrahi to Britain even if he was captured by Libyan rebels or American special forces.

The admission came amid some speculation that the bomber may be seized and passed to Nato forces during the ongoing conflict in Libya.

This weekend marks the second anniversary of the release of Megrahi who was freed from prison after a medical assessment that he only had a few months to live after being diagnosed with advanced prostate cancer. He was released after serving just eight years of a 27-year sentence for killing 270 people in the Lockerbie airline bombing.

Yesterday, the Prime Minister's spokesman was asked if the second anniversary would lead to a renewed push to return Megrahi to prison.

"I don't think there is any mechanism by which he can be brought back to the United Kingdom," he said.

The Daily Telegraph understands that the British Government believes that the Scottish authorities would have had to have led any attempt to bring back Megrahi. However, the Scottish authorities have repeatedly defended the decision to release the prisoner and say that the "due and proper process" was followed.

The Libyan Government insisted last week that Megrahi's condition had recently taken "another turn for the worse" after doctors discovered a growth on his neck.

A spokesman for Mr Cameron stressed that the focus of British efforts in Libya was to stop Colonel Gaddafi.

"We have seen some progress over the weekend. Clearly that is welcome," he said.

"We have been saying for some time that we think the Nato operation is proving successful in eroding Gaddafi's ability to wage war against his own people.

"We are going to continue to apply that pressure and we will be patient and persistent in doing so until Gaddafi stops brutalising his people."